The invention relates generally to processes for forming polycrystalline semiconductor matter or material such as silicon. More particularly, the invention concerns preparation of low-temperature polysilicon films usable as thin film transistors (ITFs) for active matrix liquid crystal displays.
Solid phase crystallization (SPC) of amorphous silicon (a-Si) films typically requires relatively high temperatures and relatively long crystallization times. The grain size of SPC films is typically in the range of about 0.3-0.5 xcexcm.
It is desirable to enlarge the grain size in such films, and several proposals have been made that involve ion-implantation. However, such conventional approaches have been unsatisfactory because they result in impractical, commercially unacceptably long crystallization times.
It is also desirable to increase the speed of crystallization upon annealing of the film. Conventional approaches to accomplish such increased speed propose including in the overall crystallization process, the formation of so-called seeds in the as-deposited film. However, such approaches require relatively low deposition rates, in the range of about  less than 20 xc3x85/minute, for the as-deposited a-Si film which limits throughput to commercially unacceptable levels.
Until now, no semiconductor crystallization process has been proposed that combines relatively fast crystallization at low temperatures with resulting, relatively large grain size for crystallized silicon particles in the polysilicon film. Desired, relatively fast crystallization is in the range of  less than 36 hours (and preferably  less than 20 hours) at 600xc2x0 C. for a 1,000 xc3x85 silicon film.
Accordingly, it is a principal object of the present invention to overcome the drawbacks and/or ineffectiveness of conventional processes.
Yet another object is to provide a process of forming polysilicon films that includes a crystallization-completing annealing step with a shorter durational requirement than conventional processes, and with a temperature requirement of xe2x89xa6600xc2x0 C.
Another object is to provide such a process that produces polysilicon films with a relatively large grain size of greater than about 0.5 xcexcm.
Yet another object is to provide such a process that produces polysilicon films having relatively uniform grain-size distribution.
It is also an object of the invention to provide such a process that can be cost-effectively commercially practiced.
In brief summary, one aspect of the invention includes a solid-phase crystallization process of forming a film of polycrystalline semiconductor matter from a film containing the matter in an amorphous state. The process includes depositing on a substrate a film of substantially amorphous matter, modulating formation of crystals in the amorphous matter, and annealing of the film to at least substantially form modulated polycrystalline matter.
The modulating step preferably includes the substeps of partially crystallizing the amorphous matter to form in the film regions of microcrystllite matter particles adjacent regions of noncrystallized matter particles, and implanting in the film ions of a preselected type, and at a preselected energy, dose and of a preselected species, selectively to eliminate preselected ones of the microcrystallite matter particles, and further to amorphize the non-crystallized regions.
Another aspect of the invention includes a solid-phase crystallization process of forming a film of polycrystalline semiconductor matter from a film containing the matter in an amorphous state, which process includes the steps of depositing on a substrate a film of substantially amorphous matter, beginning formation of crystals in the amorphous matter, interrupting the formation of crystals after a preselected time period, and annealing of the film to at least substantially form polycrystalline matter.
Preferably, the beginning step of the process includes the substep of partially crystallizing the amorphous matter to form in the film regions of microcrystallite matter particles adjacent regions of noncrystallized matter particles, and the process further includes implanting in the film ions of a preselected type, and at a preselected energy, dose and species, selectively to eliminate preselected ones of the microcrystallite matter particles, and further to amorphize the non-crystallized regions.
Yet another aspect of the invention is a crystallization process for producing a film of polycrystalline material from a film containing the material in an amorphous state. The process includes the steps of depositing on a substrate a substantially amorphous film of the material, and partially crystallizing the amorphous material to form in the film microcrystallite material particles adjacent noncrystallized regions. The process also includes implanting in the film ions of a preselected type, and at a preselected energy, dose and species, selectively to eliminate preselected ones of the microcrystallite material particles, and further to amorphize the non-crystallized regions. As used herein, amorphize means to break down crystal structure. Ultimately, an annealing of the film is performed to at least substantially form polycrystalline material.
The ion-implanting step preferably involves silicon or germaninm ions, in a preselected dose, and using an implanting energy of a preselected amount. The preselected dose and energy for silicon is a dose of 1xc3x971016 cmxe2x88x922 at an energy of less than about 100 keV, and for germanium is a dose of 1xc3x971015 cmxe2x88x922 at an energy of less than about 200 keV. An inducing substep is also performed as part of the partially crystallizing step, causing formation of microcrystallite silicon particles by heating the film for a preselected induction period. The overall process temperature is preferably 600xc2x0 C. The inducing substep preferably includes heating the film at 600xc2x0 C. in the range of about 8.5-9.0 hours, and the overall partially crystallizing step includes heating the film at 600xc2x0 C. in the range of about 12-15 hours. In other words, the partially crystallizing step includes heating the film for about 3.5-6.5 hrs. in addition to the inducing substep which includes heating the film for 8.5-9.0 hrs.
Another feature of the invention is a method of preparing a TFT film of polycrystalline material from a film containing the material in an amorphous state. That preparation method includes the above-described steps. As used herein, TFT film means the active layer of a conventional TFT device such as those TFT devices discussed in U.S. Pat. Nos. 5,391,508, 5,395,804, 5,403,756, 5,054,887 and 4,991,939, each of which patents is incorporated herein by reference.
Another feature of the invention is a low-temperature process for enhanced crystallization of partially annealed PECVD a-Si films by selective annihilation, or substantial destruction, of seeds through ion implantation and subsequent annealing.